Biaxially oriented, laminated polyester film

Abstract: A biaxially oriented, laminated polyester film has a lamination structure with at least two layers. At least one of the outermost layers of the film is a thin laminated layer A having a thickness of not more than 3 µm. In the layer A, (a) internally formed particles (and optionally mono-dispersed particles) are present, (b) internally formed particles and specified agglomerated particles (and optionally mono-dispersed particles) are present, or (c) mono-dispersed particles are present at a very small content. When the polyester film is used as a base film for magnetic recording media, the abrasion resistance of the film surface at a high speed can be greatly increased.

Description: The present invention relates to a biaxially oriented, laminated polyester film particularly suitable as a base film for video tapes. As a biaxially oriented polyester film used as a base film for magnetic recording media, a film containing spherical silica particles originating from colloidal silica is known and described in, for example, JP-A-SHO 59-171623. Further, as a biaxially oriented, laminated polyester film, a polyester film, wherein a thin layer containing particles to form surface protrusions is laminated on a base layer, is also known and described in, for example, JP-A-HEI 2-77431. In such conventional polyester films, however, there is a problem that a film surface, particularly, protrusions formed on the film surface, is likely to be abraded by a contacting roll or guide or an edge of an applicator, this being particularly so nowadays with the recent increase in tape or film running speed in FF and REW modes of a video tape recorder, in calendering or otherwise during processing for making magnetic recording media, in dubbing processes, or in processes for incorporating a tape into a cassette. Further, there is a problem that the signal/noise ratio (S/N) when the film is processed into a video tape is not so good and it is difficult to obtain a sufficiently high-quality image when the original video tape formed from the film is dubbed particularly at a high dubbing speed which has been realized by high-speed magnetic field transferring technology. It would be desirable to provide a biaxially oriented, laminated polyester film whose surface is hardly abraded even in a high-speed running in a video tape recorder or in a high-speed process for processing the film as described above, and to provide a biaxially oriented, laminated polyester film which is suitable as a base film for video tapes wherein reduction of image quality at the time of dubbing can be suppressed when the film is formed as a video tape, that is, a good electromagnetic conversion property can be obtained. A polyester film according to the present invention has a laminate structure with at least two layers, at least one layer of which laminate structure is biaxially oriented. At least one outermost layer of the film is a thin laminated layer A having a thickness of not less than 0.01 µm and not more than 3 µm. At least the thin layer A contains (1) internally formed particles (that is, non-incorporated particles) are prepared by adding at least one of a calcium compound, a magnesium compound, a manganese compound and a lithium compound soluble in the glycol, and a phosphate and/or an ester compound thereof in an appropriate manner (a) in a process of condensation polymerization after a direct esterification of the dicarboxylic acid and ethylene glycol or (b) in a process of condensation polymerization after a transesterification of the dimethylester of a dicarboxylic acid and ethylene glycol and (2) particles of at least one type selected from (A) agglomerate particles having a mean degree of agglomeration of 5 to 100 at a content of not more than 2%, preferably 0.01-2%, by weight, each of the agglomerate particles being formed from particles having a mean primary particle diameter of 5 to 200 nm and (B) mono-dispersed particles having a mean degree of agglomeration of less than 5 and a mean primary particle diameter of not less than 0.05 µ m and not more than 3 µ m at a content of not more than 3% by weight. When such a biaxially oriented, laminated polyester film is used as a base film for magnetic recording media, the abrasion resistance of the film surface at a high speed can be greatly increased. Embodiments of the present invention will now be explained in more detail. We have found that an abrasion test determined using a razor at a high film-running speed is suitable as an index for determining whether a film surface is easily abraded or hardly abraded in a high-speed process. Further, we have also found that a low-speed razor abrasion test wherein a film is run on a razor at a low speed can become an index for determining the abrasion resistance of the film surface, although it is slightly poor in accuracy as compared with the high-speed razor abrasion test. The low-speed razor abrasion test can be performed more simply and safely. The methods for performing these tests are described later in more detail. As a result of investigation of the abrasion resistance of various films by the above-described two tests, we have found that films capable of achieving the following performance levels when tested as described above are capable of satisfactory abrasion resistance in practical use. Namely, in order to achieve a level of abrasion resistance such that problems are not encountered in a high-speed process, the level determined by the above-described high-speed razor abrasion test must be not more than 180 µ m. Further, in the low-speed razor abrasion test, the level must be not more than 40 µm. A methods for achieving these levels of not more than 180 µm in the high-speed razor abrasion test and not more than 40 µm in the low-speed razor abrasion test is as follows. A laminate portion of a biaxially oriented laminated film is formed as a thin layer, while generating internally formed particles in the thin laminated layer. External particles the type defined about one above added at a very small content into the above thin laminated layer with the internally formed particles therein to form surface protrusions and thereby enlarge the intervals of the protrusions. In any case, the polyester film according to the present invention must have a laminate structure with at least two layers. If the film is a single-layer film, it is impossible or difficult to control both surfaces to provide optimum conditions different from each other. In the present invention, by forming a laminate structure with two or more layers, for example, the surface of the layer A can be controlled to provide a good slipping surface and the surface of another outermost layer can be controlled to provide a flat surface. When a laminate structure has three or more layers, an intermediate layer can be composed of a recycled polymer to contribute to saving or recycling of natural resources. Further, in the present invention, the thickness of at least one of the outermost layers of the film (layer A) must be not less than 0.01 µ m and not more than 3 µ m. By forming such a thin laminated layer A, particles present in the layer do not sink deeply into the interior of the film and protrusions having a uniform height can be formed on the film surface. The above-described method which provides a film embodying the invention will be explained in more detail. A biaxially oriented polyester film used as a base film of a video tape generally contains particles added to it to form protrusions on the surface thereof in order to reduce the friction of the film surface. Among the protrusions thus formed, excessively large protrusions are likely to be broken in a high-speed process. The fragments generated by the break of the protrusions cause dropout at the time of recording or regenerating a video tape. Materials forming such excessively large protrusions are, for example, particularly large mono-dispersed particles among added particles, agglomerated particles formed by agglomeration of added particles and contaminated materials in the polymer preparing process. Further, when external particles such as spherical silica particles are used as the particles to be added to a film, because they are poor in affinity with the matrix polymer forming the film, voids are liable to be generated around the particles. Therefore, when an impact is applied to the protrusions formed, the particles present beneath the protrusions are likely to fall off. Accordingly, in order to prevent such defects, protrusions are formed by internally formed particles. The internally formed particles are very good in affinity with the base material, and can form surface protrusions while achieving a condition of strong bonding with the base material. By allowing such internally formed particles to be present in the thin laminated layer A, broad protrusions having a uniform height can be formed on the film surface. In this case, the external particles may be present in a small amount together with the internally formed particles, as needed. The protrusions formed by the external particles, which are present between the protrusions formed by the internally formed particles, are protected in a high-speed process by the broad protrusions formed by the internally formed particles, and as a result, the abrasion of the protrusions formed by the external particles and the falling of the external particles can also be prevented. Thus, external particles other than internally formed particles may be contained in the thin laminated layer A. The abrasion resistance at high speed and the electromagnetic conversion property of the film can be further improved by using other appropriate particles together. As the external particles added to the layer A, agglomerate particles (A) having a mean degree of agglomeration of 5 to 100 each of which agglomerate particles is formed from particles having a mean primary particle diameter of 5 to 200 nm, preferably 10 to 100 nm are preferred. They are present in an amount of at least 0.01% and not more than 2% by weight, preferably 0.05 to 2% by weight, still more preferably 0.1 to 1% by weight. If the content is more than 2% by weight, there is a possibility that excessively large particles having too large a degree of agglomeration are generated. As the kind of particles, for example, alpha-type, gamma-type, delta-type or theta-type alumina particles, zirconia particles, silica particles and titanium particles can be used. These particles do not form a substantial number of protrusions on the film surface, and are added mainly in order to reinforce the skin of the surface. External particles, which can be used either alternatively or additionally to the above particles and which are added for the purpose of reducing the friction of the film surface by forming protrusions among external particles added to the layer A, are mono-dispersed particles (B) having a mean degree of agglomeration of less than 5 and a mean primary particle diameter of 0.05 to 3.0 µ m, preferably 0.1 to 2 µ m, more preferably 0.2 to 1 µ m. Examples of the kind of such particles (B) are calcium carbonate particles, mono-dispersed silica particles, divinylbenzene particles, silicone particles, titanium oxide particles, silicon carbide particles, silicon nitride particles, titanium nitride particles and tin oxide particles. The content thereof is not more than 3% by weight, preferably in the range 0.01 to 3% by weight, more preferably in the range of 0.05 to 3% by weight, and even more preferably in the range of 0.05 to 2% by weight. If the content is less than 0.01% by weight, the friction is likely to increase. If the content is more than 3% by weight, non-dispersed and agglomerated particles are likely to be present, and they are likely to cause an abrasion in a high speed process. In these mono-dispersed particles (B), the mean primary particle diameter "d" ( µ m) of the mono-dispersed particles and the thickness "t" ( µ m) of the thin laminated layer A preferably satisfies the equation 0.2d ≦ t ≦ 10d, and more preferably satisfies the equation 0.3d ≦ t ≦ 5d. In the film according to the present invention, mainly with respect to the formation of the protrusions on the layer A, the parameter of surface roughness (P10) is preferably not more than 350 nm, more preferably not more than 300 nm. If the parameter is more than this value, there is a fear that the abrasion resistance at high speed and the electromagnetic conversion property may decrease. Further, as to the height of the surface protrusions of the layer A, the relative standard deviation of height distribution thereof is preferably not more than 1.2, more preferably not more than 1.0. If the relative standard deviation is more than this value, there is a fear that undesired excessively large protrusions may be generated; such protrusion portions are likely to be abraded, thereby reducing the abrasion resistance at high speed, as well as decreasing the electromagnetic conversion property. Furthermore, in the film according to the present invention, the other surface layer B is not particularly restricted, and the constitution of the layer B can be decided depending upon the requirements for the surface of the layer B. For example, in a case where the surface of the layer B is required to be formed as flat as possible, the content of particles (external particles and/or internally formed particles) in this layer may be suppressed so as to be small. In such a manner, the surface of the layer A can be formed to have an excellent abrasion resistance at high speed, to form surface protrusions having uniform and desired height and to have an excellent electromagnetic conversion property, and at the same time, the surface of the layer B can be formed as a required flat surface. In the polyester film according to the present invention, at least one of the layers constituting the film must be biaxially oriented. It is particularly preferred that all layers of the lamination structure are biaxially oriented. If all layers are non-oriented or uniaxially oriented, the characteristics aimed by the present invention cannot be satisfied. Although the polyester constituting the film according to the present invention is not particularly restricted, a polyester containing at least one structural unit of ethylene terephthalate, ethylene α , β-bis(2-chlorophenoxy)ethane-4,4'-dicarboxylate and ethylene 2,6-naphthalate as its main constituent is particularly preferred. Among these, a polyester containing ethylene terephthalate as its main constituent is particularly preferred. Two or more kinds of polyester may be blended and a copolymerized polymer may be used, unless the desired properties are significantly impaired. In the film according to the present invention, other polymers may be blended as long as the desired properties are not significantly impaired. Furthermore, a generally used organic additive such as an oxidation inhibitor, thermal stabilizer, lubricant or ultraviolet absorbent may be added in the volume usually employed. The intrinsic viscosity (IV) of the polymer of the layer A of the laminated polyester film according to the present invention is preferably in the range of 0.4 to 0.9 to obtain a further excellent abrasion resistance at high speed. Moreover, the difference between the polymer IV of the layer A and the polymer IV of the layer B is particularly preferably not more than 0.1 to obtain a further excellent abrasion resistance at high speed. Next, a process for producing the film according to the present invention will be explained in more detail. However, the process is not particularly, restricted to this. Firstly, as a process for preparing internally formed particles, the following process is effective. Namely, the internally formed particles are prepared by adding at least one of a calcium compound, a magnesium compound, a manganese compound and a lithium compound soluble in the glycol, and a phosphate and/or an ester compound thereof in an appropriate manner (a) in a process of condensation polymerization after a direct esterification of the dicarboxylic acid and ethylene glycol or (b) in a process of condensation polymerization after a transesterification of the dimethylester of a dicarboxylic acid and ethylene glycol. In the addition of the compound for forming the internally formed particles, it is preferred that at least one of a calcium compound and a lithium compound is added to the reaction system as a glycol solution at an appropriate timing from the time when the esterification or the transesterification has substantially finished until the time when the condensation polymerization has not yet proceeded so much. As the calcium, magnesium, manganese and lithium compound soluble in glycol, for example, an inorganic salt such as a halide, a nitrate or a sulfate, an organic salt such as an acetate, an oxalate or a benzoate, a hydride or an oxide, is preferably employed, and two or more compounds may be used together. As the phosphoric compounds, at least one of phosphoric acid, phosphorous acid and esters or partial esters thereof can be employed. As the method for controlling the content of particles, a method for preparing a particle master having a high concentration by the above method and diluting the particle master with a polyester which contains substantially no particles when the film is produced is effective. In a method for preparing a film embodying the present invention, the concentration of the particle master is preferably in the range of 0.5 to 5% by weight. Next, a polyester film having a two or more layer laminate structure is formed using the pellets of the polyester thus prepared. The pellets of polyester prepared in the above manner are blended at a predetermined ratio, and after drying, they are supplied to known extruders for melt-extruding and lamination, molten polymers are delivered out from a die in the form of a sheet and the sheet is cooled and solidified on a casting roll to form a non-stretched film. Namely, the lamination is carried out by using two or more extruders and a die having a manifold or a feed block for two or more layers (for example, a feed block having a rectangular lamination portion), a molten sheet having two layers is delivered out from a die and the delivered sheet is cooled on a casting roll to form a non-stretched film. In such a manner, it is effective to install a static mixer and a gear pump in the polymer path. Furthermore, it is effective to set the temperature of the extruder for extruding the thin laminated layer A lower than the temperature of the other extruder, for example, lower by 5 to 10 °C. Next, the non-stretched film obtained is biaxially stretched and biaxially oriented. As the stretching process, a sequentially biaxial stretching or a simultaneously biaxial stretching can be employed. In particular, a process for using a sequentially biaxial stretching, wherein firstly longitudinal stretching and secondly transverse stretching are carried out, is preferred, and a process for dividing the longitudinal stretching into three or more stages and setting the total draw ratio of the longitudinal stretching to 3.5 to 6.5 times is especially preferred. Although it is difficult to determine the optimum temperature for the longitudinal stretching to a particular temperature because the optimum temperature varies depending on the kind of polyester, usually setting the temperature of the first stage to 50 to 130 °C and the temperatures of other stages after the first stage to a higher temperature is effective. The stretching rate in the longitudinal direction is preferably in the range of 5,000 to 50, 000%/min. The transverse stretching is usually conducted using a stenter. The draw ratio of the transverse stretching is preferably in the range of 3.0 to 6.0 times. The stretching rate in the transverse direction is preferably in the range of 1,000 to 20,000%/min, and the temperature for the stretching is preferably in the range of 80 to 160°C. The biaxially stretched film thus formed is then heat treated. The temperature of the heat treatment is preferably in the range of 170 to 220 °C, more preferably in the range of 180 to 200°C, and the time of the heat treatment is preferably in the range of 0.2 to 20 seconds. Next, methods for determining the characteristics of films of the present invention and estimating the effects achievable according to the present invention will be explained. (1) Mean primary particle diameter of particles present in film: Polymer is removed from the film by plasma low temperature ashing treatment to expose the particles. The condition of the ashing treatment is selected such that the particles are not damaged whereas the polymer is ashed. The exposed particles are observed by a scanning electron microscope (SEM) and the image of the particles is processed by an image analyzer. The magnification of the SEM is about 2,000 to 10,000 times, and a visual field per one determination is appropriately selected from the range of one side length of 10 to 50 µ m. Changing the portions to be observed, the volume mean diameter "d" of the particles of not less than 5,000 is determined from the diameters and volume rate thereof by the following equation. d = Σ d 1 · Nvi Where, d 1 is diameter of particle, and Nvi is volume rate thereof. In a case where the particles are organic particles and the particles are greatly damaged by the plasma low temperature ashing treatment, the following method may be employed. Using a transmission type electron microscope (TEM), the cross section of the film is observed at a magnification of 3,000 to 100,000 times. The thickness of a cut sample for the measurement of the TEM is set to 1,000 Å, not less than 500 fields are measured changing the fields, and the volume mean diameter "d" of the particles is determined from the above equation. (2) Content of particles: The film is treated with a solvent which dissolves polyester but which does not dissolve the particles, and the particles are separated from the polyester by centrifugation. The content of the particles is defined as the ratio of the weight of the separated particles to the total weight of the film (% by weight). Alternatively, as needed, the determination may be carried out by utilizing infrared spectrophotometry. (3) Lamination thickness of film: The depth profile of the particle density is determined by X-ray photoelectron spectrum (XPS), infrared spectroscopic analysis (IR) or a confocal microscope while etching from the surface. At the surface of the surface layer of a laminated film, the particle density is low because the surface is an interface, and the particle density increases as the measuring position is apart from the surface. In the film laminated with a layer according to the present invention, the particle density becomes the maximum at a depth [I] and thereafter decreases again. Based on this distribution curve of the particle density, a depth [II] which indicates 1/2 of the maximum of the particle density is defined as the lamination thickness (where, II > I). Further, in a case where inorganic particles are contained, using an apparatus of secondary ion mass spectrum (SIMS), the ratio of the density of the element originating from the particles present at the highest density in the film to the density of the carbon in the polyester (M + /C + ) is defined as the particle density ratio, and the determination is conducted from the surface of the layer A in the depth direction (thickness direction). The lamination thickness is determined in a manner similar to the above-described manner. The lamination thickness ca also be determined by observation of the cross section of the film or using a thin film step measuring apparatus. (4) Mean degree of agglomeration of particles: The film containing particles is cut in the direction perpendicular to the surface plane of the film to make a very thin piece having a thickness of 1,000 Å. When the thin piece is observed at a magnification of about 100,000 times using a transmission type electron microscope (for example, JEM-1200EX: produced by Nippon Denshi Corporation, a Japanese company), a minimum particle which cannot be further divided (primary particle) can be observed. This observation is carried out on 20 fields of view, the number of primary particles forming each agglomerated particle is counted, and the mean value of the number is defined as the mean degree of agglomeration. (5) Relative standard deviation of height distribution of surface protrusions: The height data of the protrusions measured with a two beam and two detector type scanning electron microscope (ESM-3200; produced by Elionics Corporation) and a cross section measuring apparatus (PMS-1; produced by Elionics Corporation) by scanning them along the surface of the film setting the flat portion of the film surface as the base (height 0) is transferred to an image processor (IBAS-2000; produced by Karl Zeiss Corporation), and an image of the protrusions on the film surface is reconstructed on the display of the image processor. Then, a circle equivalent diameter of a protrusion is determined from the area of each protrusion obtained by processing the protrusion portion reconstructed on the display in a binary condition, and the determined circle equivalent diameter is defined as a mean diameter of the protrusion. Further, the highest value among the height data of the respective protrusion portions of a protrusion processed into a binary condition on the display is defined as a height of the protrusion, and this determination is repeated on the respective protrusions. The determination is repeated 500 times changing measuring portions, and in a condition where the height distribution of the determined protrusions is supposed to indicate a normal distribution whose center is on a point of height "0", the standard deviation is determined by least square approximation. The relative standard deviation of the height distribution of the particles is determined as the ratio of the obtained standard deviation to the mean height of the particles. The magnification of the scanning electron microscope is set in the range of 1000 to 8000 times. (6) Parameter of surface roughness P10: Using a photo-interference type three-dimensional surface analyser (TOPO-3D; produced by WYKO Corporation, magnification of objective lens; 40-200 times, use of a high resolution camera is effective), the image of the protrusions of the film surface is constructed on the display of an image processor. The mean value of ten points from the highest protrusion portions of the image constructed on the display is defined as P10. The measurement is repeated twenty times and the mean value thereof is defined as the parameter of surface roughness P10 (unit: nm). (7) Intrinsic viscosity of polymer (IV): The intrinsic viscosity of polymer is determined using o-chlorophenol as the solvent at 25°C. (8) Abrasion resistance at high speed (high-speed razor abrasion test): The film is slit into a tape having a width of 1/2 inch. The tape is run by 200m under a condition where a safety razor is pressed onto the tape so that the knife edge is positioned lower by 0.5 mm than the level of the tape applied with a tension (running speed: 200 m/min, tension: 100g) in a direction perpendicular to the tape surface. The height of the powder on the razor, which is cut off from the tape by the razor and adhered to the razor, is determined by a microscope, and the height is defined as the abraded amount ( µ m). If the abraded amount is not more than 180 µ m, the abrasion resistance is determined to be "good", and if the abraded amount is more than 180 µ m, the abrasion resistance is determined to be "not good". (9) Abrasion resistance at low speed (low-speed razor abrasion test): The film is slit into a tape having a width of 1/2 inch. The tape is run by 10 cm under a condition where a safety razor is pressed onto the tape so that the knife edge is positioned lower by 1.0 mm than the level of the tape applied with a tension (running speed: 3.3 cm/min, tension: 100g) in a direction perpendicular to the tape surface. The height of the powder on the razor, which is cut off from the tape by the razor and adhered to the razor, is, determined by a microscope, and the height is defined as the abraded amount ( µ m). If the abraded amount is not more than 40 µ m, the abrasion resistance is determined to be "good", and if the abraded amount is more than 40 µ m, the abrasion resistance is determined to be "not good". (10) Electromagnetic conversion property: A magnetic coating solution having the following composition is coated with a gravure roll on the surface of the film so that the coated magnetic layer is magnetically oriented, and then it is dried. After calendering the film by a small test calendering machine (steel roll/nylon roll; 5 stages) at a temperature of 70 °C and a linear pressure of 200 kg/cm, the film is cured at 70°C for 48 hours. The film obtained is slit to make a pancake of a tape having a width of 1/2 inch. The tape of 250m from the pancake is incorporated into a VTR cassette to make a VTR cassette tape. (Composition of magnetic coating solution) (All parts are by weight.) Co-containing iron oxide 100 parts Vinyl chloride/vinyl acetate copolymer 10 parts Polyurethane elastomer 10 parts Polyisocyanate 5 parts Lecithin 1 part Methylethylketone 75 parts Methylisobutylketone 75 parts Toluene 75 parts Carbon black 2 parts Lauric acid 1.5 parts 100% chromatic signal generated by a television testing wave generator is recorded in the tape using a domestic VTR, and chromatic S/N is determined from the regenerated signal using a color video noise measuring apparatus (unit: dB). EXAMPLES AND COMPARATIVE EXAMPLES Preferred embodiments of the invention will be hereinafter explained with reference to Examples and comparative Examples. The results of these Examples and comparative Examples are shown in Tables 1 to 3. Example 1 (Table 1) Polyethylene terephthalate pellets (50 parts by weight) containing each of internally formed and external particles and polyethylene terephthalate pellets (50 part by weight) containing no particles were blended, and the blended pellets were supplied to a vent-type twin screw extruder No. 1 to melt the pellets at a temperature of 280°C (polymer I). Polyethylene terephthalate pellets containing calcium carbonate particles having a mean particle diameter of 0.8 µ m at a content of 0.2% by weight were dried at a temperature of 180°C for 3 hours in a pressure-reduced condition (3 Torr), and thereafter the pellets were supplied to another extruder No. 2 to melt the pellets at a temperature of 290°C (polymer II). After these two kinds of polymer were filtered at a high accuracy, the polymers were laminated in a feed block having a rectangular lamination portion for two-layer lamination structure such that the polymer II is placed at a position of layer B and the polymer I is placed at a position of layer A, and the laminated polymers were delivered out from a fish-tail type die in the form of a sheet. The delivered sheet was cast on a casting drum having a surface temperature controlled at 30°C and cooled and solidified on the drum using an electrostatic charging method to form a non-stretched film having a thickness of about 160 µ m. The draft ratio between the die and the casting drum was 6.5. The non-stretched film was stretched in the longitudinal direction at three stretching stages wherein the first stage was at 123°C and 1.2 times, the second stage was at 126°C and 1.45 times and the third stage was at 114°C and 2.3 times. The uniaxially stretched film obtained was stretched in the transverse direction using a stenter at two stretching stages wherein the first stage was at 111°C and 3.7 times and the second stage was at 113°C and 1.2 times, and then the film was heat treated under a condition of a constant length of 200°C for five seconds to make a film having a thickness of 14.5 µ m. The ratio of lamination thicknesses (layer A/layer B) of the obtained biaxially oriented film was 2.5/12 ( µ m). The parameter of surface roughness (P10) thereof was 190, and the relative standard deviation of height distribution of the protrusions formed on the film surface was 0.9. As the result of the estimation of the properties of the film and the video tape made therefrom, as shown in Table 1, the electromagnetic conversion property was 2.1 dB, the abrasion resistance at high speed of the surface of the layer A was 130 µ m, and the abrasion resistance at low speed thereof in the lowspeed razor abrasion test was 18 µ m. Thus, when the kind of the particles present in the layer A and the laminated structure satisfy the conditions specified by the present invention, excellent electromagnetic conversion property and abrasion resistance at high speed can be achieved. Examples 2-4, Comparative Example 1 (Table 1) Biaxially oriented polyester films were prepared in a manner similar to that of Example 1. In Comparative Example 1, however, the film was a single layer film. As in Example 1, in Examples 2-4, external particles belonging to the particles (A) according to the present invention were added to the laminated layer A in addition to the internally formed particles. In Examples 2-4, each of which satisfies the conditions specified by the present invention, excellent electromagnetic conversion property and abrasion resistance at high speed could be obtained. Examples 5-11 (Table 2) Biaxially oriented, laminated polyester films were prepared in a manner similar to that of Example 1. In these Examples, external particles belonging to the particles (B) according to the present invention were added to the laminated layer A in addition to the internally formed particles. In Examples 5-11, each of which satisfies the conditions specified by the present invention, excellent electromagnetic conversion property and abrasion resistance at high speed could be obtained. Examples 12-15, Comparative Examples 2-4 (Table 3) Biaxially oriented, laminated polyester films were prepared in a manner similar to that of Example 1. In these Examples and Comparative Examples, both of external particles belonging to the particles (A) and belonging to the particles (B) according to the present invention were added to the laminated layer A in addition to the internally formed particles. In Examples 12-15, each of which satisfies the conditions specified by the present invention, excellent electromagnetic conversion property and abrasion resistance at high speed could be obtained. However, in Comparative Examples 2-4, which are outside the present invention, at least one of the electromagnetic conversion property and abrasion resistance obtained were poor.

Claims: A biaxially oriented polyester laminate film having at least two layers, at least on outermost layer of which said laminate film is a thin layer A having a thickness of not less than 0.01 µm and not more than 3 µm, at least the said thin layer A containing each of: (1) internally formed particles which are obtainable by adding at least one of a calcium compound, a magnesium compound, a manganese compound and a lithium compound soluble in the glycol, and a phosphate and/or an ester compound thereof in an appropriate manner (a) in a process of condensation polymerization after a direct esterification of the dicarboxylic acid and ethylene glycol or (b) in a process of condensation polymerization after a transesterification of the dimethylester of a dicarboxylic acid and ethylene glycol; and (2) externally added particles of at least one type selected from type: (A) externally added agglomerate particles at a content of not less than 0.01% by weight and no more than 2% by weight, the agglomerate particles having a mean degree of agglomeration of 5 to 100, and each of said agglomerate particles being formed from particles having a mean primary particle diameter of 5 to 200 nm; and (B) externally added mono-dispersed particles, at a content of not more than 3% weight, the mono-dispersed particles having a mean degree of agglomeration of less than 5 and a mean primary particle diameter of not less than 0.05 µ m and not more than 3 µ m. A film according to claim 1, wherein the thickness of the said layer A is not less than 0.01 µm and not more than 2 µm. A film according to claim 1 or claim 2, which contains at least particles of type A. A film according to claim 3, wherein the mean primary particle diameter of the said primary particles forming each of the said agglomerate particles is in the range of 10 to 100 nm. A biaxially oriented, laminated polyester film according to claim 3 or claim 4, wherein the content of the said agglomerated particle in the said thin layer A is in the range of 0.05 to 2% by weight. A film according to claim 5, wherein the content of the said agglomerate particles in the said thin layer A is in the range of 0.1 to 1% by weight. A film according to any preceding claim, which contains at least particles of type B. A film according to claim 7, wherein the mean primary particle diameter of the said mono-dispersed particles is in the range of 0.1 to 2 µm. A film according to claim 8, wherein the mean primary particle diameter of the said mono-dispersed particles is in the range of 0.2 to 1 µm. A film according to any one of claims 7 to 9, wherein the content of the said mono-dispersed particles in the said thin layer A is in the range of 0.01 to 3% by weight. A film according to claim 10, wherein the content of the said mono-dispersed particles in the said thin layer A is in the range of 0.05 to 3% by weight. A film according to claim 11, wherein the content of the said mono-dispersed particles in the said thin layer A is in the range of 0.05 to 2% by weight. A film according to any one of claims 7 to 12, wherein the mean primary particle diameter "d" (µm) of the said mono-dispersed particles and the thickness "t" (µm) of the said thin layer A satisfies the equation 0.2d ≤ t ≤ 10d. A film according to claim 13, wherein the mean primary particle diameter "d" (µm) of the said mono-dispersed particles and the thickness "t" (µm) of the said thin layer A satisfies the equation 0.3d ≤ t ≤ 5d. A film according to any preceding claim, wherein the said thin layer A has a parameter of surface roughness (P10) not more than 350 nm. A film according to claim 15, wherein the parameter of surface roughness (P10) of the said thin laminated layer A is not more than 300 nm. A film according to any preceding claim, having a relative standard deviation of height distribution of protrusions formed on the surface of the said thin layer A of not more than 1.2. A film according to claim 17, wherein the relative standard deviation of height distribution of protrusions is not more than 1.0. A film according to any preceding claim, having an abrasion of the surface of the said thin layer A, determined using a razor at a low film-running speed, of not more than 40 µm. A film according to any preceding claim, having only two layers.

Inventors: ABE KOICHI, MIYAKE TOHRU, OHSHIMA KEISUKE, OKAZAKI IWAO, ABE, KOICHI, MIYAKE, TOHRU, OHSHIMA, KEISUKE, OKAZAKI, IWAO, OKAZAKI, IWAO, LIONSMANSION JYURAKUNIJYO 504
Applicants: TORAY INDUSTRIES, TORAY INDUSTRIES, INC.
Application year: 1994
Main classification: B32B27
IPC codes: G11B5, B32B27